1. Field of the Invention
The invention concerns a process for laser beam welding with prior spot welding (tack welding, riveting). A general process of this type is already known from DE 44 14 464 C1.
2. Related Art of the Invention
When welding with high power density welding processes, in particular laser beam welding, electron beam welding or plasma welding, the gap between the pieces being joined may not exceed a maximum gap width, the width being determined by the particular process and material being joined. This is of particular importance in overlap-welding of body panel sheet metal in automobile construction. For this reason, the sheets of metal must generally be pressed together or clamped very near to the welding site.
Generally, a stationary clamping technique, using a number of clamping elements, is employed for clamping, thereby ensuring that the permissible gap width is not exceeded at each point of the welding seam.
FIG. 1 shows a prior art a mobile clamping technique according to DE 198 03 734 C2. For this, a pressure element 4, which at the same time provides or feeds the welding rod or wire 13, moves over the sheets 11 to be welded and presses these together at point 14. A laser welding head 18 is rigidly connected the pressure element and is oriented rigidly relative thereto, in order for laser beam 21 to melt the welding wire. In this embodiment the clamping and welding speed are coupled and thus slow.
Modern welding processes, for example laser-scanner-welding, can achieve welding speeds that are substantially higher than the moving speed achievable by a coupled welding-clamping device. The latter is also limited by the requirement that the sheets, once aligned upon each other, should not be brought out of alignment by transmission of impulses.
In DE 44 14 464 C! it is proposed to completely dispense with clamping techniques. The sheets are basically superimposed, then riveted with multiple spot-welds using a laser along the length of the intended welding seam, and thereafter are welded.
This process may be satisfactory for flat sheets. It has been found by experience, however, that in the case of complex geometry gap-widths occur which cannot be closed without clamping techniques.